1. Field of the Invention
The present invention relates to a method of producing a superconducting wire, and more particularly it relates to a method of producing a superconducting ceramic wire having a protective metal coating thereon.
2. Related Background Art
A metal oxide ceramic superconducting wire has poor flexibility and is susceptible to quality degradation. A protective metal coating or covering has often been made on the surface of such superconducting ceramic wire.
Japanese Patent Application Publication No. 134819 (Sedaka et al) and European Patent Application Publication No. 285169 (Nakai et al) disclose the covering during so-called melt-drawing method including annealing step to crystallize the drawn wire in one process. The former deals with Y-Er-Ba-Cu-O ceramic system, and a coating is effected after annealing step. The latter discloses the covering of a variety of ceramic systems including Y-Ba-Cu-O and Bi-Sr-Ca-Cu-O systems, and the covering is effected by leading the melt into a platinum pipe acting as a support.
U.S. Pat. No. 4,975,416 (to Onishi et al) corresponding to European Patent Application No. 369464 which is incorporated herein by reference discloses a method of producing a superconducting ceramic wire of various systems including Bi-Sr-Ca-Cu-O, Bi-Pb-Sr-Ca-Cu-O, Tl-Ba-Ca-Cu-O and Tl-Bi-Sr-Ca-Cu-O systems, which is called now melt-quenching and preform wire-drawing process in which the starting oxide mixture is molten, quenched to a glass in amorphous state, a preform made of the glass is wire-drawn with keeping amorphous state to a such extremely thin wire as 100 .mu.m or less and the wire is subjected to heat-treatment to give superconductivity. According to the reference method, a superconducting ceramic thin wire having higher flexibility and better superconducting properties than those obtained by the above other methods can be realized with excellent productivity. The method with its characterization or identification is also published as articles in a Japanese journal by reporters including some of the present inventors. (Japanese Journal of Applied Physics, Vol., 28, No. 12, Dec. 1989; and Vol. 29, No. 1, Jan., 1990, which are also incorporated herein by reference).
The reference U.S. Pat. No. 4,975,416 also discloses a metal (e.g. Ag)-protecting cover of the resulting superconducting wire. According to the Patent, a wire which is super conductive is coated or covered with metal by sandwiching between metal plates, inserting in a metal pipe, dipping into a molten metal or winding a metal tape.
All the covering or coating methods disclosed in the above references are incomplete, expensive, difficult, troublesome or liable to degrade the property of wire covered.
The significance of the covering is not only to improve the poor flexibility of the superconducting wire obtained by the melt-quenching process so as not to be damaged by a slight strain, but also to improve its property.
The following drawbacks of the superconducting wire obtained by the melt-quenching method are not improved or resolved. They are:
1) The wire is so brittle that a pressure sufficient to enhance the critical current density due to the crystal orientation is differently applied.
2) A superconducting state of the superconductor fiber is partially disturbed under the influence of ambient turbulence such as in magnetic field or mechanical strain when a current flows through the fiber. Joule heat is generated by such a portion of the fiber. Since the conventional fiber has a poor heat dissipation property, the temperature of the fiber as a whole is increased to disturb the superconducting state of the entire superconducting state.
3) Some elements of the oxide superconductor material are evaporated during a heat treatment for precipitating a superconductor crystal phase. Therefore, desired properties of the superconducting fiber cannot be obtained.
It is expected that problems (1) to (3) can be overcome by forming a protective metal coating layer of, e.g., Ag on the surface of the superconducting fiber. A practical technique for coating Ag on a fiber manufactured by the prior-art method can be a plating method or a dipping method of dipping a fiber into an Ag melt. The plating method results in poor productivity. In the dipping method, the temperature of the Ag melt reaches 961.degree. C. It is impossible to dip a superconducting fiber in the Ag melt because the superconducting fiber is softened at 400.degree. to 500.degree. C.
The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.